B and T lymphocyte development is characterized by the progressive rearrangement of gene segments in progenitor cells to form the functional antigen receptors that are expressed on mature cells. The progenitor cells assemble unique stage-specific versions of these receptors, the pre-B cell receptor (pre-BCR) and pre-T cell receptor (pre-TCR). The expression of these receptor complexes is critical for the normal development of the lymphocyte lineages, since a mutation in any of their components compromises the maturation of the lineage. Nevertheless, and in spite of years of effort, the mechanism(s) by which the pre-BCR and pre-TCR function is unknown, and remains highly controversial. One model suggests that the assembly of the receptors results in the generation of a constitutive activation signal, which permits (or induces) the continued development of the lineage. Another model suggests that the receptor engages a ligand, which then activates the receptor signaling machinery, leading to the continued development of the selected cell. Circumstantial evidence supporting both hypotheses has been obtained but direct evidence for either is lacking. In this R21 application, we propose to test the hypothesis that ligands for the pre-BCR exist. We have generated a soluble form of the pre-BCR and wish to use it to test for the existence of molecules that might serve as ligands for the pre-BCR. The specific aims are: 1) To identify cells that express molecules that interact with the pre-BCR, and determine the specificity of the interaction(s); 2) To isolate and characterize the molecules bound by the receptor, as a prelude to determining their function; and 3) To determine if the soluble receptor can inhibit the interactions between progenitor B cells and ligand-expressing (stromal?) cells, in vivo or in vitro. These experiments may provide the first direct evidence for the existence of ligands for the pre-B cell receptor, and will establish the feasibility of additional studies to characterize the function of these molecules in B cell development. By analogy, these studies may also provide novel insights into T cell development.